RAIN WATER CISTERNS

RAINWATER CISTERNS

This article, written by the American Ground Water Trust was originally published in THE AMERICAN WELL OWNER, 2001, Number 4]

RAINWATER CISTERNS FOR HOMES

A rainwater storage cistern coupled with a rainfall catchment area can be an effective way to attain a source of water for a home. This type of system can provide a source of water in areas where there is infrequent rainfall, no local water supply infrastructure or where ground water is non-existent or of very poor quality. Historically many people throughout the world have relied on this kind of water supply system. For example, many homes in the Middle East and on the Island of Bermuda collect and store rainwater.

A cistern is essentially a covered tank usually made out of concrete, galvanized metal, fiberglass or polyethylene for storing water. Rainwater is collected from a catchment area such as a rooftop and channeled through gutters and collection pipes into the cistern. Catchment areas generally have smooth surfaces to increase the quality and quantity of harvested rainwater. All openings and access ports in the cistern should be screened to prevent any unwanted contamination from pollutants, animals and debris. A “first flush rejection device” should be installed before (prior to) the cistern to remove solid pollutants and other debris that is carried from the catchment surface during the first few minutes of a rain event. This first volume of water carries the largest portion of the debris and contaminants that have accumulated since the last precipitation. Approximately 10 gallons of first flush collection volume is recommended for each 1,000 square feet of collection surface feeding the cistern. An example of a “roof washer” first flush system is shown in the diagram (from the Texas Water Development Board website). The first flush is collected in the roof washer pipe volume of the system.

When sizing a cistern and catchment area, analyze the average annual rainfall and average daily amount of water a household plans to use to insure the collection of enough water. Considering precipitation type, evaporation, the efficiency of the collection method/ materials, generally, three-quarters of the rainfall volume is recoverable for use over the year. If precipitation is concentrated within a relatively short “rainy season,” be sure to plan for a water supply during the dry season, perhaps using “hauled” water or a larger storage system. A cistern should have an overflow pipe of a diameter equal to the diameter of the collection (inflow) pipe to prevent damage to the cistern when it is full.

In cold climates where the temperature frequently drops below freezing, cisterns may be located below the ground surface (e.g., below the frost line) to prevent them from freezing. In warm climates, a buried cistern may use the relatively constant ground temperature and insulating effects of the ground to maintain a water temperature that is cooler than the ambient air temperature. Underground systems tend to be more expensive because of the added excavation cost and extra reinforcement of the cistern wall that provides the support and wall strength required to prevent collapse when the tank is empty.

To maintain a safe, healthy cistern water source for drinking and washing, filter and disinfect the water prior to use. Screens and membrane cartridges filter fine particles from the water. Harvested rainwater can be disinfected with chlorination, ozonation, or ultraviolet irradiation (the most common type). An opaque storage tank is recommended to help reduce algae growth.

Periodic maintenance is essential to insure clean safe drinking water. Routinely check seals and clean or replace screens and filters. We recommend that after any work on the cistern, clean and disinfect the system.